1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave filter for use as a band filter in mobile communications equipment.
2. Description of the Related Art
Surface acoustic wave filters are more widely used as band filters in mobile communications equipment, since the filters can be reduced in size, in contrast to dielectric filters or other filters. The band filters for use in mobile communications equipment are required to have a low loss in the transmission bands. Accordingly, the surface acoustic wave filters have been variously designed and constructed to reduce the loss.
For example, a surface acoustic wave filter using one terminal-pair surface acoustic wave resonator shown in FIG. 15A has been proposed. Here, grating reflectors 202 and 203 each having a plurality of electrode fingers, are arranged on both of the sides in the surface acoustic wave propagation direction of an interdigital transducer 201. The loss in the transmission band of the one terminal pair surface acoustic wave resonator is reduced by the grating reflectors 202 and 203.
Moreover, a surface acoustic wave resonator having only one interdigital transducer 205 has been proposed as shown in FIG. 15B. Here, the number of electrodes in the interdigital transducer 205 is large, for example, 200 electrodes. Thereby, surface acoustic wave energy can be trapped in the area where the interdigital transducer 205 is located without reflectors being provided. That is, a multi-pair type energy trapping surface acoustic wave resonator is formed.
Furthermore, a plurality of interdigital transducers 206 and 207 are arranged in the surface acoustic wave propagation direction in the resonator type surface acoustic wave filter shown in FIG. 16C. Grating reflectors 208 and 209 are arranged on both sides in the surface acoustic wave propagation direction of the area where the interdigital transducers 206 and 207 are located, respectively.
Moreover, a surface acoustic wave filter having a ladder circuit configuration and a surface acoustic wave filter having a lattice circuit configuration, in each of which a combination of plural surface acoustic wave resonators is provided as described above and shown in FIGS. 15A and 15B, have been proposed.
As described above, the energy of an excited surface acoustic wave can be trapped by providing reflectors, or by increasing the number of the electrode finger pairs of an interdigital transducer. Thus, the Q value, which is a resonance characteristic, can be enhanced, and the loss can be reduced.
On the other hand, the electrode resistance of a surface acoustic wave device, the surface acoustic wave mode, the electrode capacity, and so forth are affected by the ratio of the width L1 of each electrode finger 211 in an interdigital transducer shown in FIG. 17, based on the gap size L2 between adjacent electrode fingers 211 in the surface acoustic wave propagation direction in the interdigital transducer, that is, the ratio of L1/(L1+L2) (hereinafter, referred to as duty, briefly), and moreover, the electrode film thickness h/xcex of the interdigital transducer (xcex is the wavelength of a surface acoustic wave, and h/xcex is a film thickness standardized by xcex. Thus, for design of the surface acoustic wave device, it is important to optimize these parameters.
The gap length L2 represents the distance in the surface acoustic wave propagation direction of the gap.
As described above, conventionally, surface acoustic wave filters have been variously designed so as to enhance the filter characteristics. For example, Japanese Unexamined Patent Application Publication No. 7-28368 discloses a longitudinally coupled resonator type surface acoustic wave filter using a 36xc2x0 Y-cut X-directional propagation LiTaO3 piezoelectric substrate and moreover, utilizing coupling of modes in the horizontal direction relative to the surface acoustic wave propagation path. According to this publication, the ohmic resistance loss can be reduced, and the steepness of the filter characteristic can be increased by setting the electrode film thickness of the interdigital transducer to be in the range of 0.06 xcex to 0.10 xcex, and also, setting the duty of the interdigital transducer at about 0.6 or higher.
On the other hand, Japanese Unexamined Patent Application Publication No. 6-188673 discloses a ladder surface acoustic wave filter in which plural one terminal-pair surface acoustic wave resonators are formed on a 36xc2x0 Y-cut X-directional propagation LiTaO3 substrate. FIG. 18 shows the ladder circuit. In FIG. 18, S1 and S2 represent series arm resonators, and P1 to P3 represent parallel arm resonators, respectively. In this conventional surface acoustic wave filter, the electrode film thickness h/xcex of the interdigital transducer is in the range of 0.4 xcex to 0.10 xcex, whereby an undesired spurious can be removed from the transmission band to improve the filter characteristic.
According to the above-described publications, the resistance loss can be reduced, and the spurious suppressing effect can be obtained by setting the film thickness of the interdigital transducer at 0.04 xcex or more and setting the duty at 0.5 or higher when the 36xc2x0 Y-cut X-directional propagation LiTaO3 is used.
Recently, mobile communication systems have been operated at higher frequencies, and the frequencies at which surface acoustic wave filters are operated in the systems become higher, that is, the frequencies are in the range of 800 MHz to 2.5 GHz. The acoustic velocities of surface acoustic waves are about several thousand meters per second. Thus, when a surface acoustic wave device is formed so as to operate at 800 MHz to 2.5 GHz, the wavelength of a surface acoustic wave is short, that is, about several xcexcm. Accordingly, electrode patterns for defining the interdigital transducers and the reflectors must be very fine.
Therefore, the absolute value of the electrode film thickness become small, and the width of each electrode finger become small. As a result, the loss (ohmic loss), caused by the electrode resistance, cannot be made negligible.
Moreover, when the thickness of each electrode becomes small, the strength of the electrode is reduced. Accordingly, electrodes that are capable of being wire-bonded cannot be formed.
Thus, it has been attempted that the film thickness of portions of the electrodes, such as bus bar electrodes, turning-around electrodes, and wire bonding pads, excluding the electrode portions where a surface acoustic wave is excited in practice, is increased to reduce the ohmic loss as much as possible, whereby the strength required for wire-bonding is secured.
For example, Japanese Unexamined Patent Application Publication No. 62-47206 discloses a surface acoustic wave filter in which acoustic coupling of the component of a surface acoustic wave in the vertical direction to the surface acoustic wave propagation direction is caused. As described in this publication, in this surface acoustic wave filter, the thickness of each of the bas bar electrodes shared by the interdigital transducers adjacent to each other in the surface acoustic wave propagation direction is larger than that of each electrode finger of the interdigital transducers. Thus, the acoustic velocity can be controlled while the resistance is reduced. Therefore, a desirable filter characteristic can be obtained.
In the surface acoustic wave resonators shown in FIGS. 15A and 15B and in the resonator type surface acoustic wave filter shown in FIG. 16, the energy can be trapped by increasing the number of the electrode fingers of the reflectors, and increasing the number of electrode pairs of the interdigital transducer to reflect the surface acoustic wave substantially completely. However, the surface acoustic wave has not only an X-directional component but also a component in the vertical direction to the X-direction, that is, a Y-directional component in the vertical direction to the main plane of the piezoelectric substrate. Thus, the surface acoustic wave propagates while the Y-directional component extends in a beam shape. For this reason, it is necessary to sufficiently trap the energy of the surface acoustic wave in the Y-axial direction. Unless the energy is not sufficiently trapped, the diffraction loss will increase, so that the Q value is deteriorated.
As described in the Journal of the Acoustical Society of Japan, 3-1-1, 77-78 (1979/6), the anisotropy index xcex3 is less than xe2x88x921 on a 36xc2x0 Y-cut propagation LiTaO3. The anisotropy index xcex3 is a constant in the following formula by which the acoustic velocity (xcex8), obtained when the propagation direction is deviated by an angle xcex8 from the X-axis, is expressed. In the formula, V0 is the acoustic velocity when xcex8 is 0xc2x0.
V(xcex8)=V0xc3x97(1+xcex3/2xc3x97xcex82)
In the case in which the anisotropy index xcex3 is less than xe2x88x921, the energy is trapped when the velocity in the wave guide is lower than that outside the wave guide. That is, Vs/Vm greater than 1 is the condition required for energy trapping, in which Vs is the velocity of a surface acoustic wave in the area where the electrode fingers are provided, and Vm is the velocity of the surface acoustic wave propagating on each bus bar electrode.
On the other hand, it has been found that the ratio Vs/Vm, that is, the ratio of Vs representing the velocity of a surface acoustic wave propagating on the area where the electrode fingers are meshed with each other, to Vm representing the velocity of the surface acoustic wave propagating on each bus bar electrode is significantly varied depending on the duty and the electrode film thickness, when the film thickness of the electrode fingers and that of the bus bar electrode are equal to each other.
In particular, when the electrode film thickness is small, and the duty is low, the ratio Vs/Vm greater than 1 is satisfied. When the electrode film thickness and also the duty are increased, the ratio Vs/Vm is decreased. The ratio Vs/Vm reaches Vs=Vm on a certain condition. When the duty or the electrode film thickness is further increased, the ratio Vs/Vm becomes less than 1. That is, substantially no energy can be trapped in the Y-axial direction.
FIG. 19 shows a relationship between the electrode film thickness h/xcex and the ratio Vs/Vm, obtained when the interdigital transducer made of Al is formed on a 36xc2x0 Y-cut X-directional propagation LiTaO3 substrate, and the duty is 0.5. As seen in FIG. 19, the ratio Vs/Vm has a maximum value when the electrode film thickness h/xcex is in the range of 3% to 4%, namely, in the range of 0.03 to 0.04. When the electrode film thickness h/xcex become larger, the ratio Vs/Vm is decreased, changing along the parabolic curve. Especially, it is observed that the ratio Vs/Vm is rapidly decreased when the electrode film thickness h/xcex exceeds 0.06 xcex.
If the length in the Y-axial direction of each bus bar electrode is infinite, the energy can be trapped, provided that the ratio Vs/Vm is less than 1. In the case in which the length in the Y-axial direction of each of the bus bars is definite, the energy trapping effect will be reduced, if the ratio Vs/Vm is not sufficiently large. Thus, the loss in the filter characteristic is increased.
FIG. 20 shows a relationship between the duty and the ratio Vs/Vm, obtained when the interdigital transducer is made of Al, and the electrode film thickness is constant, that is, 0.06 xcex on a 36xc2x0 Y-cut X-directional propagation LiTaO3 substrate.
As seen in FIG. 20, when the duty is low, the ratio Vs/Vm is large. As the duty is increased, the ratio Vs/Vm is reduced. Especially, when the duty exceeds 0.8, the ratio Vs/Vm becomes less than 1. Thus, the energy trapping condition is not satisfied.
Furthermore, the following Table 1 shows change of the ratio Vs/Vm, obtained when the duty and the electrode film thickness are varied.
As seen in TABLE 1, as the film thickness and the duty are increased, the ratio Vs/Vm is decreased. Especially, the ratio Vs/Vm is less than 1 when the relationships satisfying the following formulae (1) to (6) are obtained, that is, in the conditions where the values listed in the columns on the right side from the thick lines in TABLE 1 can be obtained. Thus, the wave mode in the Y-axial direction cannot be satisfied substantially.
L1(L1+L2)xe2x89xa70.55 and h/xcexxe2x89xa70.100xe2x80x83xe2x80x83(1)
L1(L1+L2)xe2x89xa70.60 and h/xcexxe2x89xa70.090xe2x80x83xe2x80x83(2)
L1(L1+L2)xe2x89xa70.65 and h/xcexxe2x89xa70.080xe2x80x83xe2x80x83(3)
L1(L1+L2)xe2x89xa70.70 and h/xcexxe2x89xa70.070xe2x80x83xe2x80x83(4)
L1(L1+L2)xe2x89xa70.75 and h/xcexxe2x89xa70.065xe2x80x83xe2x80x83(5)
L1(L1+L2)xe2x89xa70.80 and h/xcexxe2x89xa70.055xe2x80x83xe2x80x83(6)
In the surface acoustic wave resonators shown in FIGS. 15A and 15B and in the resonator type surface acoustic wave filter shown in FIG. 16, the electrode resistance loss can be reduced, and an undesirable spurious can be eliminated by increasing the electrode film thickness and also the duty. This was estimated to be preferable.
Referring to the energy trapping effect in the Y-axial direction of the surface acoustic wave, the trapping effect becomes maximum at an electrode film thickness of 0.04 xcex, and is reduced when the electrode film thickness becomes 0.04 xcex or more.
Moreover, similar phenomena are observed when the duty is increased. The energy trapping effect is reduced at a duty of 0.5 or higher.
Especially, the energy trapping condition cannot be satisfied in the range where the relationship between the electrode film thickness and the duty fulfills a certain condition. Thus, the loss in the filter characteristic is increased.
Accordingly, it is preferable that the electrode film thickness is up to 0.04 xcex, and the duty is up to 0.5 to obtain the greatest energy trapping effect.
However, when the electrode film thickness is small, and the duty is 0.5 or less, the filter characteristic is deteriorated for a reason other than the above-described one, as seen in the above-described Japanese Unexamined Patent Application Publication No. 7-283682 and the Japanese Unexamined Patent Application Publication No. 6-188673.
In other words, the optimum electrode structure of a surface acoustic wave filter for obtaining the preferred filter characteristic thereof and the optimum electrode structure from the standpoint of the above-described energy trapping effect in the Y-axial direction are different from each other. Both of the electrode structures have a trade-off relationship.
Moreover, Japanese Unexamined Patent Application Publication No. 62-47206 describes that the acoustic coupling degree between the interdigital transducers can be enhanced, and the bandwidth can be increased by increasing the thickness of each bus bar electrode shared by interdigital transducers adjacent to each other in the surface acoustic wave propagation direction to be larger than that of each electrode finger, until the acoustic velocity Vs of the surface acoustic wave propagating on the electrode fingers is equal to the acoustic velocity Vb of the surface acoustic wave propagating on each bus bar electrode.
The above-described phenomena are caused in the configuration of the surface acoustic wave filter in which the interdigital transducers are acoustically coupled to each other perpendicularly to the surface acoustic wave propagation direction. When Vs is equal to Vb, the above-described energy trapping effect in the Y-axial direction is reduced to the contrary.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device which can efficiently trap the energy of an excited surface acoustic wave, and moreover, can reduce the loss and improve the filter characteristic.
According to a first preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate at which a surface acoustic wave is excited, having an anisotropy index xcex3 in the propagation direction of less than about xe2x88x921, and at least one interdigital transducer disposed on the piezoelectric substrate, having a plurality of electrode fingers each containing Al as a major component and first and second bus bar electrodes, in which the energy of the surface acoustic wave is trapped substantially perpendicularly to the propagation direction of the surface acoustic wave, the electrode fingers each having a film thickness of not less than about 0.04 xcex in which xcex is the wavelength of the surface acoustic wave, at least a portion of the first and second bus bar electrodes having a thickness larger than that of each electrode finger.
According to a second preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate at which a surface acoustic wave is excited, having an anisotropy index xcex3 in the propagation direction of less than about xe2x88x921, and at least one interdigital transducer disposed on the piezoelectric substrate, having a plurality of electrode fingers each containing Al as a major component and first and second bus bar electrodes, in which the energy of the surface acoustic wave is trapped substantially perpendicularly to the propagation direction of the surface acoustic wave, the electrode finger width L1 of the interdigital transducer and the gap length L2 between adjacent electrode fingers in the surface acoustic wave propagation direction satisfying the formula of L1/(L1+L2)xe2x89xa70.5, at least a portion of the first and second bus bar electrodes having a thickness larger than that of each electrode finger.
According to a third preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate at which a surface acoustic wave is excited, having an anisotropy index xcex3 in the propagation direction of less than about xe2x88x921, and at least one interdigital transducer disposed on the piezoelectric substrate, having a plurality of electrode fingers each containing Al as a major component and first and second bus bar electrodes, in which the energy of the surface acoustic wave is trapped substantially perpendicularly to the propagation direction of the surface acoustic wave, the film thickness h1 of each electrode finger, the electrode finger width L1, the gap length L2 between adjacent electrode fingers in the surface acoustic wave direction, the wavelength xcex of the surface acoustic wave satisfying one of the following formulae (1) to (6);
xe2x80x83L1(L1+L2)xe2x89xa70.55 and h/xcexxe2x89xa70.100xe2x80x83xe2x80x83(1)
L1(L1+L2)xe2x89xa70.60 and h/xcexxe2x89xa70.090xe2x80x83xe2x80x83(2)
L1(L1+L2)xe2x89xa70.65 and h/xcexxe2x89xa70.080xe2x80x83xe2x80x83(3)
L1(L1+L2)xe2x89xa70.70 and h/xcexxe2x89xa70.070xe2x80x83xe2x80x83(4)
L1(L1+L2)xe2x89xa70.75 and h/xcexxe2x89xa70.065xe2x80x83xe2x80x83(5)
L1(L1+L2)xe2x89xa70.80 and h/xcexxe2x89xa70.055xe2x80x83xe2x80x83(6)
at least a portion of the first and second bus bar electrodes having a thickness larger than that of each electrode finger.
Preferably, at least a portion of the first and second bus bar electrodes have a multi-layer structure in which a plurality of electrode films are laminated to each other, whereby at least a portion of the first and second bus bar electrodes have a thickness larger than that of each electrode finger.
Also, preferably, in each bus bar electrode having a multi-layer structure, the electrode film defining the lowest layer is arranged so as to be connected to the electrode fingers, respectively, and the electrode films defining the second and the succeeding layers are made of a metal different from that used to form the electrode film defining the lowest layer.
Also, preferably, in each bus bar electrode having a multi-layer structure, at least one layer of the electrode films defining the second and the proceeding layers is made of a metal having a relatively high density compared to the electrode film defining the lowest layer.
Also, preferably, in each bus bar electrode having a multi-layer structure, at least one layer of the electrode films defining the second and the proceeding layers has a lower resistivity and a larger thickness compared to the electrode film defining the lowest layer.
Preferably, in each bus bar electrode having a multi-layer structure, an insulating film is disposed between electrode films constituting the multi-layer structure so as to secure electrical connection between the upper and lower electrode films.
Moreover, preferably, in each bus bar electrode having a multi-layer structure, the distance g of the boundary between the bus bar electrode and the electrode fingers in the electrode film of the lowest layer to the edge on the electrode finger side of the electrode film made of Al defining the second layer, and the film thickness M of the electrode film defining the second layer are in the range determining by the formula of Mxe2x89xa70.159gxe2x88x920.094, in which values of g and M are integral multiples of the wavelength xcex of the surface acoustic wave.
Also, preferably, in each bus bar electrode having a multi-layer structure, the film-thickness Ma of the second layer is in the range defined by the formula Maxc3x97(d0/da)xe2x89xa70.159gxe2x88x920.094, in which g is the distance from the boundary between the bus bar electrode and the electrode fingers to the edge on the electrode finger side of the electrode film defining the second layer, Ma is the electrode film thickness of the second layer, the values of g and M are expressed by integral multiples of the wavelength xcex of the surface acoustic wave, respectively, the second layer is made of metal excluding Al, da is the density of the meal of the second layer, and d is the density of Al.
According to a fourth preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate at which a surface acoustic wave is excited, having an anisotropy index xcex3 in the propagation direction of less than about xe2x88x921, and at least one interdigital transducer disposed on the piezoelectric substrate, having a plurality of electrode fingers each containing Al as a major component and first and second bus bar electrodes, in which the energy of the surface acoustic wave is trapped substantially perpendicularly to the propagation direction of the surface acoustic wave, the film thickness of the electrode fingers in the interdigital transducer being not less than about 0.04 xcex, in which xcex is the wavelength of the surface acoustic wave, an insulating film being disposed on each bus bar electrode so that the thickness of the bus bar electrode is larger than that of each electrode finger.
According to a fifth preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate at which a surface acoustic wave is excited, having an anisotropy index xcex3 in the propagation direction of less than about xe2x88x921, and at least one interdigital transducer disposed on the piezoelectric substrate, having a plurality of electrode fingers each containing Al as a major component and first and second bus bar electrodes, in which the energy of the surface acoustic wave is trapped substantially perpendicularly to the propagation direction of the surface acoustic wave, the electrode finger width L1 and the gap length L2 between adjacent electrode fingers in the surface acoustic wave propagation direction satisfying the formula of L1/(L1+L2)xe2x89xa70.5, an insulating film being disposed on each bus bar electrode so that the thickness of the bus bar electrode is larger than that of each electrode finger.
According to a sixth preferred embodiment of the present invention, a surface acoustic wave device including a piezoelectric substrate at which a surface acoustic wave is excited, having an anisotropy index xcex3 in the propagation direction of less than about xe2x88x921, and at least one interdigital transducer disposed on the piezoelectric substrate, having a plurality of electrode fingers each containing Al as a major component and first and second bus bar electrodes, in which the energy of the surface acoustic wave is trapped substantially perpendicularly to the propagation direction of the surface acoustic wave, the film thickness h1 of each electrode finger, the electrode finger width L1, the gap length L2 between adjacent electrode fingers in the surface acoustic wave direction, the wavelength xcex of the surface acoustic wave satisfying one of the above-described formulae (1) to (6), and further including an insulating film disposed on the bus bar electrode.
Preferably, the surface acoustic wave device according to various preferred embodiments of the present invention further includes an insulating film disposed on the electrode fingers, whereby the thickness of each bus bar electrode portion including the insulating film is larger than the electrode finger portion including the insulating film.
Also, preferably, the piezoelectric substrate at which a surface acoustic wave can be excited, having an anisotropy index xcex3 in the propagation direction of less than about xe2x88x921 is preferably a LiTaO3 substrate at which a pseudo surface acoustic wave can be excited, for example, a 36xc2x0 Y-cut X-directional propagation LiTaO3 substrate.
According to another preferred embodiment of the present invention, an antenna sharing device includes at least one of the surface acoustic wave devices according to the above-described preferred embodiments of the present invention.
In addition, according to yet another preferred embodiment of the present invention, a communications equipment apparatus includes at least one antenna sharing device according to the preferred embodiment described in the preceding paragraph.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.